1. Field of the Invention
The present invention relates to a photomask blank and a photomask for use in photolithography. More specifically, it relates to a photomask blank and photomask which are well suited for use in the microfabrication of high-density semiconductor integrated circuits such as large-scale integration (LSI) and very large-scale integration (VLSI) chips, charge-coupled devices, color filters for liquid-crystal displays, and magnetic heads.
2. Prior Art
Photolithographic processes involving the use of photomasks are employed in the fabrication of LSI and VLSI chips, charge-coupled devices, color filters for liquid-crystal displays, and magnetic heads. Photomasks used for this purpose are produced from a photomask blank composed of a transparent substrate such as silica glass or aluminosilicate glass on which a generally chromium-based light-shielding film has been formed by a sputtering or vacuum evaporation technique. The photomask is created by forming a specific pattern in the light-shielding film of the photomask blank.
Patterning is carried out by applying a photoresist or an electron-beam resist to the photomask blank composed of a chromium-based light-shielding film on a substrate, selectively exposing the resist to a specific light pattern, and subjecting the resist to development, rinsing and drying steps so as to form a resist pattern. Using the resulting resist pattern as a mask, the unmasked areas of the chromium-based film are removed, either by wet etching with an etchant composed of an aqueous solution of both cerium ammonium nitrate and perchloric acid, or by dry etching with a chlorine-containing gas. The resist is then removed to give a photomask having a specific pattern composed of light-shielding areas and light-transmitting areas.
The chromium-based light-shielding film has a high light reflectance. To prevent light which reflects back from the semiconductor substrate being exposed and passes through the projection lens from being reflected again by the photomask and returning to the semiconductor substrate, an antireflective coating is generally formed on the surface side, or both the surface and back sides, of the light-shielding film.
Photomasks and photomask blanks having such an antireflective coating are described in the prior art. For example, JP-B 62-37385 discloses a photomask blank comprising a transparent substrate on which have been successively formed a chromium carbide nitride film containing chromium carbide and chromium nitride as a back antireflective coating, a chromium film as a light-shielding film, and a chromium oxide nitride film containing chromium oxide and chromium nitride as a surface antireflective coating. The prior art also teaches the use of CrON (JP-B 61-46821 and JP-B 62-32782) as the antireflective coating, and the use of chromium (JP-B 61-46821), CrC (JP-B 62-27387) or CrN (JP-B 62-27386 and JP-B 62-27387) as the light-shielding film.
In addition, photomask blanks have been developed for commercial use in which a chromium-based film is formed over a halftone film in order to increase resolution.
An important requirement of photomasks is that the substrate be flat to assure accurate transfer of the pattern. Yet, no matter how flat a substrate is used, forming two, three or more chromium-based films on the substrate increases film stress within the plurality of layers, causing the substrate to warp following film formation and thus lowering surface flatness. Moreover, even if a substrate in which the surface flatness has changed due to stresses within the applied films is flat after the films have been formed, subsequent patterning and selective removal of the chromium-based films alters the flatness of the substrate and causes it to warp. As a result, when a specific mask pattern on such a photomask is transferred onto a workpiece such as a silicon substrate, pattern distortion arises.
Lately, as the level of integration and miniaturization of semiconductor integrated circuit devices has continued to rise and the geometries of circuit patterns formed on semiconductor substrates have become ever smaller, rapid advances are being made in achieving smaller feature sizes in photomask patterns as well. One serious and unwelcome consequence is that substrate warp due to film stress following film formation can result in pattern formation on the workpiece at a position other than that intended. The incidence of such positional deviation due to warping of the photomask substrate rises as the minimum feature size of pattern becomes smaller, and is an especially grave problem with very small patterns.
It is therefore an object of the invention to provide a high-performance photomask blank comprising a transparent substrate on which have been formed a light-shielding film and an antireflective film, in which photomask blank the substrate does not warp due to film stress following film formation, thus enabling a desired finely featured pattern to be formed thereon accurately and without distortion. Another object of the invention is to provide a photomask fabricated by patterning the foregoing blank.
The inventor has found that, in photomasks or photomask blanks which comprise a transparent substrate having thereon at least one light-shielding film and at least one antireflective film, by forming each film from a CrCO layer, a CrCON layer, or a combination of CrCO and CrCON layers, these CrCON layers and CrCO layers have smaller film stresses than conventional chromium films, resulting in minimal change in substrate warp after formation of the light-shielding film and the antireflective film relative to before film formation. Hence, photomask blanks and photomasks having a high degree of surface flatness can be obtained.
Accordingly, the invention provides a photomask blank comprising a transparent substrate and, on the substrate, at least one light-shielding film and at least one antireflective film; wherein each film is composed of a CrCO layer, a CrCON layer, or a combination of CrCO and CrCON layers.
According to one preferred embodiment of the invention, the photomask blank has, in order from the substrate side, a CrCO layer as the light-shielding film and a CrCON layer as the antireflective film.
According to another preferred embodiment, the photomask blank has, in order from the substrate side, a CrCON layer as the light-shielding film and a CrCON layer as the antireflective film.
According to yet another preferred embodiment, the photomask blank has, in order from the substrate side, a CrCON layer as a first antireflective film, a CrCO layer as the light-shielding film and a CrCON layer as a second antireflective film.
According to still another preferred embodiment, the photomask blank has, in order from the substrate side, a CrCON layer as a first antireflective film, a CrCON layer as the light-shielding film, and a CrCON layer as a second antireflective film.
Preferably, the light-shielding film and antireflective film have a combined film stress of not more than 0.2 GPa. Moreover, the substrate in the photomask blank of the invention undergoes a change in warp following deposition of the light-shielding film and the antireflective film, relative to before film deposition, of preferably not more than 0.2 xcexcm.
The invention also provides a photomask fabricated by lithographically patterning the foregoing photomask blank.
The inventive photomask blank and photomask which comprise, on a transparent substrate, at least one light-shielding film and at least one antireflective film, each of which is composed of a CrCO layer, a CrCON layer, or a combination of CrCO and CrCON layers, have very low film stresses. Hence, the substrate does not warp following formation of the light-shielding film and the antireflective film, making it possible to accurately form without distortion a desired pattern of small feature size. As a result, the photomask blanks and photomasks according to the invention are fully capable of meeting the technical demands of even higher levels of integration and miniaturization in semiconductor integrated circuit devices.
The CrCO and CrCON layers in the invention can be deposited using as the sputtering gas CO2, which is safer than CO. Because CO2 gas has a lower reactivity than other gases such as oxygen gas, it can spread uniformly through much of the chamber, thus helping to achieve CrCO and CrCON layers of uniform quality.